In milling cutters having shim plates that are arranged under replaceable milling inserts, the individual shim plate has an important function in preventing—or at least as far as possible counteracting—serious damage in the event of an insert breakdown. Namely, if the co-operating milling insert suddenly would lose its cutting capacity during operation, e.g., as a consequence of fractures or other unexpected faults, the basic body of the milling cutter may dig into the workpiece and cause expensive damage not only to the proper basic body but also to the driving spindle and the parts of the machine tool co-operating with the same. For this reason, the shim plate is usually mounted in such a way that its edge portion, positioned rotationally behind the chip-removing cutting edge of the milling insert, on one hand protrudes radially a distance in relation to the envelope surface of the basic body, but, on the other hand, is located inside the swept area described by the cutting edge during the rotation of the milling cutter. During normal conditions, i.e., as long as the milling insert is in working order, the peripheral edge portion of the shim plate clears the generated surface in the workpiece, at the same time as the envelope surface of the basic body positioned rotationally behind the shim plate is situated radially inside the imaginary circle described by the edge portion of the shim plate. Therefore, if an insert breakdown would occur, the peripheral edge portion of the shim plate can proceed to remove chips from the workpiece without the rotating basic body digging into the workpiece. In other words, the edge portion of the shim plate can passably assume the chip-removing function of the milling insert during the short time that is required to interrupt the milling operation before a more extensive tool and machine breakdown occurs.
Another function of the shim plate is to form a reliable and accurately located long-term bottom support for the milling insert. For this reason, the shim plate is usually manufactured from a material, such as cemented carbide, that is harder than the material of the basic body, which in turn most often consists of steel or aluminium. The shim plate is connected semi-permanently with the basic body, usually via a tubular screw that includes, on one hand, a male thread, which is tightened in a female thread in a hole that mouths in a bottom of the seat of the milling insert, and on the other hand a female thread, in which a male thread of a tightening screw can be tightened to fix the milling insert. In contrast to the shim plate—which under good conditions can function over the entire service life of the basic body without needing to be replaced—the milling insert is replaced on repeated occasions. In order to avoid a non-uniform wear of the upper side or support surface of the shim plate, this is face ground at the same time as the under side of the milling insert is allowed to protrude radially a short distance (0.1-0.2 mm) outside the radially outer edge portion of the shim plate. In such a way, it is avoided that the great number of milling inserts gradually coin and deform the upper side of the shim plate. Using cemented carbide in the shim plate also entails the advantage that the heat dissipation from the milling insert is accelerated because the coefficient of heat conductivity of the cemented carbide is greater than of the steel.
In such milling operations, where pockets or countersinks are to be created in a surface of a workpiece and where the milling cutter cannot enter the workpiece from the side, so-called ramping has to be resorted to. This means that the milling cutter is subjected to not only a transversal feeding motion, but also an axial, which implies that the milling cutter during its rotation moves obliquely downward in the material. In this connection, there arises—depending on the ramping angle in question—a risk of a phenomenon, which by those skilled in the art is denominated “back-cutting”, and which means that the inactive (and maybe unused) cutting edge positioned radially inside the zero point of the milling insert and the clearance surface connected thereto unintentionally contact the material of the workpiece and are worn out by the same. Even if back-cutting at times may give rise to wear damage already to such milling cutters that are equipped with single-sided milling inserts, the risk of damage becomes more frequent and more serious when the milling inserts are double-sided and invertible. In such cases, not only the inactive cutting edge that is situated radially inside the active chip removing cutting edge and in the same upper side as the same, but also an analogous cutting edge along the under side of the milling insert may be spoiled, if the clearance surface of the milling insert interferes with the material of the workpiece, because an axially running surface layer along the clearance surface is scraped off and leaves indentations in the respective cutting edges. If back-cutting is considerable, damage arise not only to the inactive cutting edge and the analogous cutting edge along the under side of the milling insert, but also to the basic body, which leads to the basic body, which is the most expensive part of the tool, becoming unusable.
It should also be mentioned that ramping motions between a milling cutter and a workpiece may be intentional, as well as, unintentional. Intentional ramping takes place with the conscious object of creating a countersink in the surface of the workpiece. Via the machine in question, the milling cutter is then subjected to an axial force component or feeding motion, in addition to the transversal one. In such cases, the milling cutter is most often specially designed for the object, more precisely by being equipped with single-sided milling inserts having a comparatively large functional clearance behind the inactive cutting edge and/or the cutting edge portion that is lying between the active and the inactive cutting edge. In addition, however, also unintentional ramping may be present as a phenomenon, namely if the workpiece is unstably clamped. On that occasion, the proper workpiece may be set in motion, which however are small, but which nonetheless mean that the milling cutter off and on dives down into the surface material.
An aspect of the disclosure is the task to allow ramping operations by such milling tools that are designed with shim plates with the traditional object of preventing damage to the basic body of the tool in the event of insert breakdowns. More precisely, the aspect aims at protecting the milling cutter body and the unused and inactive cutting edges that have not been utilized for chip removal and in such a way guaranteeing an optimum serviceability of the tool in general and of all cutting edges after indexing also in those cases where intentional or unintentional ramping motion arises.
According to the aspect, the above-mentioned object is attained by the shim plate being formed with a protuberance, in which there is included an auxiliary cutting edge that is delimited between a chip surface and a clearance surface and that is located in the area of the inactive cutting edge of the milling insert and projects laterally in relation to the same.
In one embodiment, the auxiliary cutting edge of the shim plate has a positive cutting geometry by the clearance surface thereof forming an acute angle in relation to the upper side of the shim plate, such as this is defined by a flat support surface located on a reference plane. In such a way, a good functional clearance is guaranteed between the auxiliary cutting edge and the machined material surface.
In yet an embodiment, the chip surface of the auxiliary cutting edge may extend from an outer cutting edge line to a ridge having a crest situated on a higher level than the support surface, the ridge, via a declining flank surface, transforming into a countersink having a bottom situated on a level below the support surface. Said ridge forms a protection also for downwardly facing, inactive cutting edges of double-sided milling inserts, more precisely by the chips removed by the auxiliary cutting edge being guided away from the inactive cutting edge that should be protected.
In another embodiment, the chip surface of the auxiliary cutting edge may be cross-sectionally concavely arched. Such a chip surface provides for a quick convolution of the removed chips, something which significantly contributes to guiding away the chips from the milling insert.
Advantageously, the auxiliary cutting edge has a length that amounts to at least ⅛ of the total perimeter of the shim plate. In such a way, it is guaranteed that the free-cutting becomes sufficiently large to protect the inactive cutting edge of the milling insert along the portion thereof positioned farthest axially away from the basic body. In such a way, the inactive cutting edge is protected at least in connection with moderate cutting depths. In order to protect the inactive cutting edge of the milling insert also at greater cutting depths, the length of the auxiliary cutting edge can be increased to approx. ¼ of the perimeter of the shim plate.
In an embodiment preferably intended for contour milling, the milling insert has a round basic shape, the auxiliary cutting edge of the shim plate having a chip surface with a crescent-like contour shape that is determined by an arched cutting edge line. By said contour shape of the auxiliary cutting edge, it is gained that the inactive cutting edge of the milling insert is protected in a reliable way without the protuberance, in which the auxiliary cutting edge is included, needing to be made so large that it disturbs the milling operation.
In yet an embodiment, the milling insert is of a polygonal basic shape and includes straight cutting edges having flat clearance surfaces, the auxiliary cutting edge of the shim plate likewise being straight by being delimited by a flat side surface. In such a way, inactive cutting edges of milling inserts for face milling or end milling can be protected in an effective way before indexing.
By utilizing a shim plate available for traditional objects for protecting the basic body and inactive, unused cutting edges if the milling tool is subjected to (intentional or unintentional) ramping. Usually, shim plates for milling cutters are manufactured of the same or similar hard materials as of the milling insert, in particular cemented carbide. By forming such a cemented carbide plate with an auxiliary cutting edge and locating the same adjacent to the cutting edge of the milling insert inactive for the time being, not only the basic body can generally be protected, but also the cutting edge inactive for the time being in order to be preserved intact for a coming indexing.